1. Field
The present invention relates to data communication, and more particularly to techniques for searching and acquiring a gated pilot reference in a wireless communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system such as increased system capacity
In a wireless communication system, a pilot reference is often transmitted from a transmission source to a receiver device to assist the receiver device perform a number of functions. The pilot reference is typically generated based on a known data pattern (e.g., all zeros) and using a known signal processing scheme (e.g., covered with a particular orthogonal code and spread with a known PN sequence). The pilot reference may be used at the receiver device for synchronization with the timing and frequency of the transmission source, estimation of the quality of the communication link, and coherent demodulation of a data transmission. For a CDMA system that conforms to IS-856 standard, the pilot reference is also used to determine the particular access point having the best link to the terminal and the highest data rate supported by this access point.
In IS-95 CDMA systems, a pilot reference is continuously transmitted from each base station on a dedicated pilot channel at a relatively low transmit power level. A terminal receives and processes a forward link signal to isolate the pilot channel, and further processes the pilot channel to recover the pilot reference. The continuous pilot transmission scheme used for IS-95 CDMA systems is more suitable for a low data rate transmission system whereby the terminal is allowed more time to process the pilot reference. However, for a high-speed data transmission system in which the link condition is required to be accurately estimated within a short time period, this continuous low-level pilot reference is not preferred.
Newer generation CDMA systems (such as those that conform to cdma2000, IS-856, and W-CDMA standards) transmit a gated pilot reference at a high (e.g., peak) transmit power level. The short but high-power pilot bursts allow a terminal to receive the pilot reference within a short time interval, which then allows the terminal to estimate the link condition within a shorter time period.
The search for a gated pilot reference is associated with greater ambiguity than that for a continuous pilot reference. To search for a continuous pilot reference, the received signal for a particular time window may be searched, and the pilot reference and its timing is determined by evaluating a number of hypotheses for all possible time offsets of the pilot reference (described in further detail below). If the pilot reference is continuous, it is present in any time window to be searched and only its timing is unknown. However, for a gated pilot reference that is not transmitted at all times, a number of time windows may need to be searched to catch the pilot reference. A search for a pilot reference over a given time window would result in a pilot acquisition failure even if all hypotheses are evaluated if the pilot reference was not transmitted during this time window. Thus, acquisition of a gated pilot reference is more complicated because a number of time windows as well as a number of hypotheses need to be evaluated to find and acquire the gated pilot reference.
There is therefore a need in the art for techniques to more efficiently search and acquire a gated pilot reference.
Aspects of the invention provide techniques to search and acquire a gated pilot reference in a manner to achieve faster acquisition time and a high probability of detection. For a wireless communication system that transmits a gated pilot reference, a larger code space needs to be searched than that for a continuous pilot reference. Various techniques are provided herein to shorten the average pilot acquisition time in the search over the larger code space.
In one aspect, the overall code space to be searched to acquire the pilot reference is divided into code sets, with each code set including a number of hypotheses to be search. The code sets are then grouped such that the resultant groups include code sets of varying likelihood of yielding a successful pilot acquisition. The groups of code sets are then search, one group at a time, in a defined order such that code sets more likely to result in successful pilot acquisition are searched first, and code sets least likely to result in successful pilot acquisition are searched last. Mechanisms are provided to terminate the acquisition process early upon successful acquisition of the pilot reference in the more likely used code space.
In another aspect, a multi-step pilot acquisition scheme is provided to search for and acquire pilot references, and some of the steps are pipelined to shorten the search time. A multi-stage pilot search scheme is also provided to search for pilot references using multiple (e.g., two) stages, wherein a different set of search parameter values may be used for each stage to improve the likelihood pilot acquisition for different operating conditions. The multi-stage pilot search scheme may advantageously incorporate the search by groups and the pipelining described above.
A specific embodiment of the invention provides a method for searching for a gated pilot reference in a wireless communication system. In accordance with the method, an overall code space in which the pilot reference may be found is initially partitioned into a number of (e.g., four) groups of non-overlapping code sets, with each code set being representative of all possible chip offsets of a specific PN sequence (with a particular PN offset). The groups are then ordered based on the likelihood of detecting the pilot reference in each of the groups, with the first group being the most likely to be used to generate the pilot reference and a last group being the least likely to be used. The groups of code sets are then used to search for the pilot reference, one group at a time, starting with the first group and ending with the last group. Upon acquisition of the pilot reference, the search may be terminated.
The search for the pilot reference may be performed using three substages comprised of a detect, a dwell, and a pull-in substage. In the detect substage for a particular group, the pilot reference is searched for in a set of samples based on the code sets in the group to provide a number of detected peaks. In the dwell substage, the detected peaks from the detect substage are re-evaluated to provide one or more candidate peaks. And in the pull-in substage, a lock is attempted on each candidate peak to achieve acquisition of the pilot reference. The detect substage for one group may be performed in parallel (i.e., pipelined) with the pull-in substage for another group.
The invention further provides other methods, receiver units, and other elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.